In many applications, particularly winch drives and traction drives driven by hydraulic motors, the load may be overrunning and cause the hydraulic motor to exceed maximum allowable speed and/or cavitate resulting in loss of control of the load or a runaway condition.
It is, therefore, desirable to have some form of automatic protection against overspeed. A commom method is employment of a so-called counterbalance valve. Use of such a counterbalance valve also requires use of a relief valve in parallel for over pressure protection. This constitutes a cumbersome and expensive solution, and it is the purpose of this invention to provide a simple and inexpensive circuit for solving the problem of overrunning loads.
In the copending U.S. application Ser. No. 024,058, filed Mar. 26, 1979, now U.S. Pat. No. 4,201,052, and Ser. No. 117,936, filed Feb. 4, 1980 having a common assignee with the present application there is disclosed hydraulic circuits wherein a valve assembly comprising a pilot operated meter-in valve and pilot operated meter-out valve is mounted directly on an actuator.
The present invention is particularly directed to the control of overrunning loads in such a hydraulic circuits.
In accordance with the invention, the pressure in the line supplying fluid to the actuator is sensed and if there is a pressure drop caused by a runaway load the meter-out valve controlling flow out of the actuator is closed thereby controlling the speed of the actuator, preventing cavitation and resultant loss of the load.